1. Field of the Invention
The present invention relates to a semiconductor module having an antenna element therein. More particularly, this invention is concerned with a semiconductor module in which an antenna element suitable for a frequency band of millimeter waves ranging from 30 gigahertz to 300 gigahertz and a frequency band of quasi millimeter waves which is lower than the frequency band of millimeter waves is incorporated.
2. Description of the Related Art
Transmitters and receivers using millimeter waves (30 GHz to 300 GHz) are needed for radar sensors to be incorporated in a motorcar or for short-distance high-speed communications over LANs or the like. A transmitter for transmitting millimeter waves included in a radar system comprises a semiconductor device (semiconductor circuit) serving as a signal source for generating millimeter waves, a mixer semiconductor device for adding information to millimeter waves, and an antenna for radiating millimeter waves. In a receiver, a millimeter-wave receiving antenna, an amplifier semiconductor device for amplifying received millimeter waves, and a mixer semiconductor device for extracting information are employed.
In the past, a plurality of semiconductor devices each of which is fabricated separately by packing components except an antenna element in a package or housing and then sealing the package or housing have generally been employed in radio communication using frequencies lower than those of quasi millimeter waves. External terminals formed on the housings of the individual devices are linked by a cable, whereby a sub system of a radio communication system is constructed. When a cable and connectors are used to connect a microwave circuit and antenna element, there arises a problem that a signal deteriorates during transmission. In particular, millimeter waves suffer from a large loss while propagating on a dielectric substrate made of alumina or the like. As disclosed in, for example, Japanese Unexamined Patent Publication No. 56-17842 or 57-44241, a transmission line based on a waveguide is often employed. However, the waveguide requires precision machining of metallic materials and does not therefore contribute to realization of a compact and inexpensive system.
As far as the frequency band of millimeter waves, for example, a frequency of 30 GHz, is concerned, since the dimension of an antenna element itself is 1 cm, the size of a radio-frequency radiation window becomes 3 cm. The size of the antenna does not become very critical. Sealing an antenna element and millimeter-wave circuit integrated into one module is therefore preferable for constructing a compact and inexpensive system because it obviates the necessity of using a cable, connectors, and a waveguide.
Japanese Unexamined Patent Publication No. 63-316905 has disclosed a structure using no waveguide. According to the disclosed structure, an antenna and semiconductor modules for transmission and reception can be linked without the use of a cable, connectors, or a waveguide. However, generally, a millimeter-wave circuit is formed on a GaAs semiconductor substrate and an antenna element is formed on a dielectric substrate made of a ceramic or the like. These substrates must be affixed to each other. The heat conductivity of the ceramic substrate on which the antenna element is formed is lower by one decade or more than that of copper or tungsten. The heat dissipation of semiconductor devices constituting a millimeter-wave circuit is a problem.
For solving such a problem, it is conceivable to arrange an antenna element and semiconductor circuit on the same surface of one metallic block so as to produce a compact and inexpensive semiconductor module for handling millimeter waves. However, even this structure has various drawbacks depending on how it is utilized actually. The first problem is directivity. For example, radio waves to be radiated from an antenna element are radiated to spread at a relatively large angle. This poses a problem that as a distance gets longer, it becomes impossible to transmit radio waves of a sufficient strength. Providing a larger output for solving this problem merely leads to an increase in circuit scale and is difficult to do from the viewpoint of radio-wave interference. It is also conceivable to improve directivity by employing an array antenna made by arranging a plurality of antenna patterns. This merely leads to an increase in size of a module. Besides, there arises a problem of a loss suffered by a signal during transmission to an antenna element. On the contrary, it may be necessary to radiate radio waves at a wider angle or receive the radio waves at a wider angle.
In relation to the foregoing problems, it becomes a problem how to efficiently radiate radio waves output from an antenna element to the outside or how to efficiently catch extraneous radio waves incident on an electromagnetic-wave radiation window using the antenna element. The efficiencies must be improved further.
In a module, an antenna element and semiconductor circuit are accommodated in a sealing cover. This poses a problem that radio waves radiated from the antenna element and those radiated from the semiconductor circuit affect each other. Moreover, the sealing cover is located closely to these components and therefore affects the components. These adverse effects produce noise to deteriorate the performance of the module.